Measuring blood pressure is an important diagnostic tool in many medical treatments, especially when treating vascular maladies. For example, aneurysms are often treated by implanting a stent-graft within the aneurysm pocket. Measuring the blood pressure at the stent-graft can be important in tracking patient health and treatment effectiveness. Various pressure sensors have been used for measuring blood pressure within a vessel, including capacitive pressure sensors.
Some of these sensors are passive inductive-capacitive (LC) sensors. A typical LC sensor connects an inductor (L) and a capacitor (C) to form an LC resonant circuit. The capacitor may be configured to vary its capacitance in response to external pressure changes, with the LC circuit's resonant frequency changing accordingly, such that the external pressure can be determined by measuring the LC sensor's resonant frequency. One advantage of a passive LC sensor is that no embedded battery is needed, since an external radio frequency (RF) energy field may be applied to the LC circuit for wirelessly sensing, where the inductive coil serves as an RF energy receiving/transmission antenna. The wireless sensing operates through magnetic induction between the sensor antenna and the external reader antenna. This magnetic induction or coupling effect increases with increasing sensor antenna size. Therefore, in general, the bigger the sensor antenna is, the deeper the sensor can operate, despite tissue absorption of RF energy.
Several significant issues rise if these LC circuits are placed within harsh environments, such as the blood stream, without sufficient protections. One issue is that water may slowly penetrate into the sensor and change the sensor's dielectric properties, which not only drifts the resonant frequency of the sensor over time, but also decreases the signal strength due to increased RF absorption loss, eventually destroying the circuit of the sensor. Another issue is that the high dielectric property of the surrounding tissue medium induces a significant parasitic capacitance to the sensor's antenna coil, and as a result, the sensor's resonant frequency may significantly shift.
To prevent the problems discussed above, a thick, water-tight electrical insulation layer could be used to encapsulate the sensor. However, using such a layer renders the sensor bulky and impedes attachment to a stent or graft. A rigid housing could also be used to encapsulate the sensor. However, a rigid housing limits the sensor's ability to flex, creating additional difficulties when inserting the sensor and/or attaching the sensor to a stent or graft. As discussed above, a wireless passive LC sensor with a large antenna exhibits increased sensing depth in the human body. However, a large rigid antenna in the sensor impedes inserting the sensor and/or attaching the sensor to a stent or graft.
While many pressure sensors incorporate an LC circuit, they rely on electrical connections between the inductor and the capacitor. However, building the LC circuit on a flex substrate, forced-folding of the sensor for interventional delivery, and/or movement of the sensor (for example due to pressure waves caused by heartbeats), places stress on those electrical connections, causing them to fail.
Thus, several needs exist in this art for a passive LC sensor that is: thin and flexible; ready to couple to a stent or graft, particularly for the placement in a vessel without blocking the blood flow; features a large effective antenna for increased sensing distance; stable when implanted in the human body; without significant signal drift over time and without significant environmental effect on sensor performance; and biocompatible and safe in the human body.